1. Field of the Invention
The present invention relates to a semiconductor laser and a method of manufacturing the same, more particularly, a semiconductor laser employed in optical fiber communication and having a buried heterostructure and a method of manufacturing the same.
2. Description of the Prior Art
As the application area of the optical fiber communication is spread from the trunk line system of the communication network to the subscriber system, an operation of the semiconductor laser as a light source is required in the wide temperature range circumstances. In particular, good laser characteristics must be attained at the high temperature at which an operating current is increased. At the same time, a required amount of the semiconductor laser is now increased.
Therefore, a structure for achieving the semiconductor laser which is operable up to the high temperature with good uniformity and a method of manufacturing the same are requested.
Normally a buried heterostructure is employed in the semiconductor laser used in the optical fiber communication. Such buried heterostructure is employed to inject a current to the active layer efficiently, and there are a buried heterostructure using a pn junction and a buried heterostructure using a semiinsulating layer. The buried heterostructure using the pn junction is suitable for the high temperature operation.
The semiconductor laser having the pn-junction buried heterostructure has a structure shown in FIG.1, for example.
In FIG.1, an active layer 2 of InGaAsP and a first p-type cladding layer 3 of p-InP are formed on an n-type InP substrate 1. Layers from the first p-type cladding layer 3 to an upper area of the n-InP substrate 1 are formed like a mesa shape to form a mesa portion. The active layer 2 in the mesa portion is formed as a stripe shape having a width of about 1 to 1.5 xcexcm. The buried heterostructures are provided on both sides of the mesa portion.
A p-type buried layer 4 of p-InP and an n-type current blocking layer 5 of n-InP are formed in the buried regions. Then, a second p-type cladding layer 6 formed of p-InP and a p-type contact layer 7 formed of p-InGaAs are formed in sequence on the n-type current blocking layer 5 and the first p-type cladding layer 3.
In addition, a p-side electrode 8 is formed on the p-type contact layer 7 and an n-side electrode 9 is formed under the InP substrate 1.
The manufacturing method of the semiconductor laser having such buried heterostructure comprises the steps of forming the buried heterostructures by growing the active layer 2 and the first p-type cladding layer 3 on the n-InP substrate 1, and forming a substntial stripe shape layers by etching from the first p-type cladding layer 3 to the InP substrate 1 by using a mask, and then forming the p-type buried layer 4 and the n-type current blocking layer 5 on both sides of the substantial stripe shape layers.
In the recent optical communication laser, a quantum well structure or a strained-layer quantum well structure is employed as the active layer in many cases. The active layer shown hereinafter means not only the quantum well structure consisting of a well layer and a barrier layer but also a structure including the quantum well structure and upper and lower light guiding layers provided to put the quantum well structure between them.
As particular reports concerning the above-mentioned structure, there are Kondo et al., 1995 Autumn Meeting the Japan Society of Applied Physics 27p-ZA-5 and Chino et al., 1997 Spring Meeting the Japan Society of Applied Physics 30p-NG-11.
However, in the buried heterostructure semiconductor laser, it is important that the leakage current which is not passed through the active layer must be reduced in order to achieve the good characteristics at the high temperature.
In the laser having a pn buried heterostructure shown in FIG.1, both sides of the active layer 2 are buried by the p-type buried layers 4, and such layers are connected to the p-type cladding layers 3, 6 formed directly on the active layer 2.
Therefore, the leakage current which flows from the p-type cladding layers 3, 6 to the n-type InP substrate 1 via the p-type buried layers 4, via routes indicated by arrows in FIG.1, is generated in the high temperature operation. Since the leakage current depends on an interval between the active layer 2 and the n-type current blocking layer 5, a distance between the active layer 2 and the n-type current blocking layer 5 must be narrowed into about 0.2 xcexcm, for example, in order to reduce the leakage current. In addition, such distance must be fabricated with good controllability in order to achieve the uniform laser characteristic.
However, in the prior art structure, an innermost point of the n-type current blocking layer 5 is set on an edge of a top surface of the mesa portion, but an angle xcex8 of the bottom surface of the n-type current blocking layer 5 spreads in the neighborhood of the active layer 2 at a gentle angle of about 30 degrees relative to the horizontal direction. Therefore, the distance between the active layer 2 and the n-type current blocking layer 5 is abruptly increased downward, so that a width of the area through which the leakage current flows is excessively widely increased.
The angle xcex8 of the bottom face of the n-type current blocking layer 5 depends on an angle of an upper surface of the p-type buried layer 4 formed under is the n-type current blocking layer 5. In other words, the (111) facet which has a slow growth rate appears at the beginning of growth in the crystal growth of the p-type buried layers 4, and then such bottom face having a gentle angle of about 30 degrees appears to start its growth on the (111) facet because of the dependence of the growth rate on facet orientation. A position and an angle of such bottom face are very sensitive to a height of the mesa portion, a lower shape of the mesa portion, change in the growth rates in respective face orientations due to the change in the growth conditions, etc.
Therefore, even if the p-type buried layer 4 is formed by the MOVPE (metal organic vapor-phase epitaxy) method which is said to have good controllability, it is difficult to fabricate uniformly the position of the n-type current blocking layer 5 with respect to the active layer 2 with good reproducibility.
It is an object of the present invention to provide a semiconductor laser capable of implementing a narrow distance between an active layer and current blocking layers formed over a substrate with good controllability, and a method of manufacturing the same.
According to the present invention, an angle of the side surfaces of the active layer which is formed on the mesa-type first cladding layer is set in the range of 70 to 90 degrees relative to the upper surface of the first cladding layer, then one end of the current blocking layer is brought into contact to an upward extension from the side surface fo the active layer, and then an angle of the facet of the current blocking layer which extends downward from the one end below the active layer is substantially inclined by 55 degrees.
Therefore, since the buried layers existing on both sides of the active layer are narrowed, the passage area for the leakage current which flows from the second cladding layer located over the active layer to the buried layer is made small to thus reduce the leakage current. As a result, the current-optical output power characteristic can be made uniform at the time of high temperature and high output power.
Such semiconductor laser manufacturing method can be attained by forming the active layer and the lower layer portion of the second cladding layer in sequence on the first cladding layer, then forming the mesa portion by patterning the layers from the lower layer portion of the second cladding layer to the upper layer portion of the first cladding layer by using the dry etching, and then forming the current blocking layer on the buried layer by controlling the growth of the buried layer such that the (111) facet exists from the side area of the active layer to the lower side thereof.
In this case, the (111) facet of the buried layer has an inclination of about 55 degrees relative to the substrate surface. In addition, formation of another facet on the (111) facet can be prevented previously by leaving the facet, which is formed in parallel with the side surface of the mesa portion, under the (111) facet of the buried layer.
Also, according to another present invention, the angle of the side surfaces of the active layer which is formed on the mesa-type first cladding layer is set in the range of 70 to 90 degrees relative to the upper surface of the first cladding layer, then one end of the current blocking layer is brought into contact to an upward extension from the side surface, and then the angle of the facet of the current blocking layer which extends downward from the one end is substantially inclined by 55 degrees, and then the angle of the other facet of current blocking layer which is formed to side of the active layer is set larger than an angle of the side surfaces of the active layer but smaller than 90 degrees on both sides of the active layer.
Therefore, since the shortest distance between the current blocking layer and the active layer can be set along overall side surfaces of the active layer, an area of the buried layer between them is narrowed. As a result, the leakage current which is passed through such area can be further reduced.
Such semiconductor laser manufacturing method can be attained by forming the active layer and the lower layer portion of the second cladding layer in sequence on the first cladding layer, then forming the mesa portion by patterning the layers from the lower layer portion of the second cladding layer to the upper layer portion of the first cladding layer by using the dry etching, and then controlling the growth of the buried layer such that the (111) facet exists over the active layer and the facet which is substantially parallel with the active layer appears below the (111) facet.
In this case, a film thickness of the buried layer between the first cladding layer and the current blocking layer must be sufficiently thick to prevent turn ON of a parasitic thyristor while reducing a film thickness of the buried layer which is to be grown on the side surface of the active layer. For this reason, there may be provided such a method that a desired film thickness is formed on the side surface of the active layer by using a chlorine containing gas in growth of the buried layer, and then an introduced amount of the chlorine containing gas is increased. Accordingly, the film thickness formed on the substrate surface can be increased locally.
By utilizing the present invention as above, a size of the buried layer in the region where the leakage current which has an influence upon the variation in the characteristics at the high temperature flows can be fabricated with good reproducibility. Therefore, the present invention can largely contribute to the highly-uniformed characteristic of the high-temperature operation optical communication semiconductor laser.